Polyurethanes have found widespread use in coatings for fabric, plastics, wood, metal and the like, because of their advantageous properties such as good chemical resistance, abrasion resistance, toughness, elasticity and durability, and their ability to cure rapidly. Such coatings are naturally applied as solutions in polar or aromatic hydrocarbon solvents, or are applied from aqueous dispersions. The employment of aqueous dispersions is gaining increasing favor in order to avoid the economic loss and the potential environmental hazards associated with the escape of solvent vapors into the atmosphere which cannot readily or economically be recaptured.
The films and coatings that are formed on substrate surfaces from solutions in organic solvents or from dispersions in aqueous vehicles tend frequently to be highly hydrophobic, even when the polyurethane polymer is configured to impart a relative degree of flexibility or resiliency to the coating. This hydrophobicity, of course, is often a very desirable property. However, there are applications in which it would be desirable that the film exhibit many of the properties for which polyurethanes are renowned, including the aforementioned resistance to chemical attack, abrasion resistance, toughness, durability and rapid cure, but in which the film or coating exhibits hydrophilicity, that is, an affinity for water or other polar and/or hydrophilic substances disposed on the cured polyurethane-based polymeric surface. Such hydrophilicity would be desired, for instance, in applications wherein the cured polyurethane polymer-based surface will be in contact (e.g. static, rolling, or sliding contact) with another surface and wherein even a minor amount of water or of another polar liquid may be present along the interface between the polyurethane polymer-based surface and the other surface. In this way, any tendency of the two surfaces to adhere to each other, where such adhesion is not desirable, could thereby be lessened or completely avoided.
One example, among many, wherein it is desirable to increase the hydrophilicity of the polyurethane polymer-based surface is in the field of rubber gloves, which are familiar adjuncts in the home, in the workplace, and in the medical professions. Gloves which are designed to fit relatively closely to the fingers, thumb and hand are relatively difficult to put on because of frictional engagement between the interior surface of the glove and the skin. This difficulty is increased when the glove is made from elastomeric material, such as natural rubber latex or other synthetic polymeric (i.e., homopolymeric or copolymeric) elastomers. This difficulty is compounded further when the hands or the interior surfaces of the glove are damp or wet. It has been conventional practice to attempt to alleviate these difficulties by employing dry particulate lubricating agents such as talc or starch which are applied to the interior surface of the glove, or to the hand, or both before the glove is donned. These agents add to the expense of the product, and they pose a risk of particles falling from the glove onto adjacent areas with consequences ranging from the catastrophic, when the glove is being worn by a surgeon in the operating theater, to the inconvenient, when the wearer is working at home or the workplace. In addition, such particulate lubricating agents do not form an effective barrier between the skin and the interior surface of the glove, whereas a barrier is highly desirable in order to protect the wearer against the risk of contacting allergenic agents known to be present in many elastomeric materials including natural rubber latex.
The present inventors have determined that the advantages of hydrophilicity can be imparted to a polyurethane polymer-based product and to films cast therefrom by certain techniques by which hydrophilicity-imparting agents are chemically incorporated into the polyurethane polymer.